The present invention relates to an image processing apparatus applicable to a copier or similar recording equipment and capable of enhancing the sharpness of characters and lines.
An image processing apparatus for the above application generally has color separation filters arranged regularly in association with CCD (Charge Coupled Device) or similar photoelectric transducer elements and processes R, G and B signals separated by such filters. It has been customary with this type of image processing apparatus to use a group of photoelectric transducer elements each being capable of converting light of particular wavelength band. Specifically, a plurality of groups of such transducer elements are arranged regularly in a unidimensional direction. When the center of a pixel to be processed is coincident with the center of a transducer element of desired wavelength (e.g. blue (B)), the output signal of such a transducer element is selected; if the former is not coincident with the latter, there is selected the sum of values inversely proportional to the intercenter distances of the outputs of transducer elements of other desired wavelength bands (e.g. red (R) and green (G)) which have the centers thereof located at both sides of the above-mentioned transducer element. This kind of scheme is disclosed in, for example, Japanese Patent Laid-Open Publication (Kokai) No. 50261/1988 and can read the position of an image with accuracy by linearly interpolating the positional deviation of dot among R, G and B by use of two pixels neighboring a pixel of interest.
However, the problem with the above-described image processing apparatus is that a weighting and averaging procedure is indispensable to implement the linear interpolation. The weighting and averaging procedure needs a digital filter having a low pass characteristic. Such a digital filter suppresses the high frequency components of an image to thereby lower the sharpness of characters and lines. For example, assuming that the center of a pixel of interest is B and positioned between R and G, R and G images are blurred. To eliminate this problem ascribable to linear interpolation, Japanese Patent Laid-Open Publication No. 126076/1989 discloses a method which interpolates by referencing data of a plurality of pixels the image data in which two colors other than a reference color have been shifted relative to the reference color by, for example, a.+-.(1/3) dot. This method, however, does not read an image located on the same position of a document and, therefore, cannot execute color correction or undercolor removal essential to, for example, a digital color copier with accuracy.